Keetch-Byram Drought Index (KBDI) is a continuous reference scale for estimating the dryness of the soil and duff layers. The index increases for each day without rain (the amount of increase depends on the daily high temperature) and decreases when it rains. This system is based primarily on recent rainfall patterns. It is a measure of meteorological drought; it reflects water gain or loss within the soil.
The scale ranges from 0 (no moisture deficit) to 800 (extreme drought). The range of the index is determined by assuming that there is 20 cm of moisture in a saturated soil that is readily available to the vegetation (Keetch and Byram, 1968). KBDI is world widely used for drought monitoring for national weather forecast, wildfire prevention and usefully especially in regions with rain-fed crops.